Metrology is the study of measuring things. You might think it’s kind of boring, but it’s extremely important. Metrologists are the ones who make sure that our measuring devices are giving us accurate values for what we need to measure. The story of the loss of the Mars Climate Orbiter is an example of how an error in measurement units can cost millions of dollars and the loss of a possibly unique opportunity.
Measurement is based upon standards. Those standards can quickly and simply give us the base units relevant to the measurement we want to make. The universal measurement system for science is the Systeme Internationale, or SI. The base units are the second (for time), the meter (for length), and the kilogram (for mass). These were all based on units originally defined, in the late 18th Century, in terms of what scientists at the time believed were invariant quantities. For example, the meter was originally defined as one ten-thousandth of the distance from the north pole to the equator, and the kilogram was defined as the mass of the volume of water that would fit in a cubic container with a 1 decimeter edge length (1 decimeter = one tenth of a meter). It was all supposed to be very consistent.
However, there were unanticipated problems with these definitions. The distance between the north pole and the equator, even through a consistent point, can vary due to tidal or geothermal forces. The density of water is a function of temperature and is sensitive to purity and other vagaries. The standards of what would become the SI needed more reliable and consistent definitions.
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In recent times, it was decided that the second would be defined as a precise number (specifically 9,192,631,770) of a certain type of vibration that occurs within the cesium-133 atom. If you have the technology to perform such a measurement, you can determine the duration of a second very precisely anywhere in the universe, as long as you have a cesium-133 atom.
The meter used to have a similar definition, but nowadays, because the measurement of the speed of light has become extremely precise, it was decided to define the meter in terms of the speed of light rather than the other way around. In 1983, the speed of light was set to exactly 299,792,458 meters per second. With the value of the second (which you got from cesium-133 atoms), and this value for the speed of light, you can determine the length of the meter anywhere in the universe.
Then there’s the kilogram, and here we have a problem. The kilogram is a hunk of platinum-iridium alloy carefully stored in a facility near Paris. There’s only one. As such, it’s called an artifact. It is not possible to determine the value of the kilogram anywhere in the universe. You have to go to Paris to do it.
[I hasten to add that each nation has its own standard kilograms which are used to calibrate instruments in that country, but these national standards must periodically be flown to Paris to determine if they still weigh a kilogram.]
In addition to the inconvenience of not having a sufficiently universal standard of mass that you could determine its value anywhere in the universe, there is another problem: the mass of the kilogram in Paris is changing, and nobody is sure why. We are relying on an artifactual standard of mass that is itself not reliable!
Over the decades, metrologists have striven to replace this artifact with a more universal definition. A few years ago, it was decided to do for the kilogram the same sort of thing that was done for the meter: set the value of a universal constant, and define the kilogram in terms of that constant. The constant in question is Planck’s constant, the quantity that relates the frequency of light to the energy of a single photon of that light. Before setting the constant to a particular value, however, it was necessary to obtain a measurement of it with minimal uncertainty. The international body in charge of these things dictated that there had to be at least two measurements of this constant with uncertainties smaller than 50 parts per billion (ppb), and at least one with uncertainty less than 20 ppb. By the deadline, July 1 of this year, three groups managed to provide measurements that satisfied the more stringent standard, including our own NIST (National Institute of Standards and Technology). Their measurement: 6.626069934 x 10-34 kilogram-square-meters per second. Now the information needed to set Planck’s constant exists, presumable in a year or two, a value will be chosen, and thereby the new definition of the kilogram. And the old “Grand K” can be thrown into the Seine.
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Top Comments, (July 20, 2017):
From susans:
This explains everything. (By nonnie9999 in wagatwe’s recommended post Japan’s first lady knows how to speak English. She just didn’t want to talk to Trump.
Top Mojo (July 19, 2017):
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Tonight’s picture quilt is courtesy of jotter!